1. Field of the Invention
The invention relates generally to a method and apparatus for removal of impurities from hydrogen. In particular, the invention relates to a method and apparatus which achieves removal of contaminants to below 1 part per billion (ppb), using a purifier containing a metal hydride getter stage operated at temperatures below 350.degree. C.
2. Brief Description of the Prior Art
Hydrogen is widely used in the semiconductor processing industry. As processing requirements become more stringent, the need for higher purity hydrogen sources increases. A variety of hydrogen purification techniques have been used. None of these methods is capable of producing purified hydrogen with sub-ppb levels of methane in the quantities required for most semiconductor manufacturing operations. (See Lorimer, D. H., "Advances in Hydrogen Purification Technologies", 1992 Microcontamination Conference Proceedings 819-825, and Ketkar, S. N., and Martinez de Pinillos, J. V., "Calibrating APIMS for H.sub.2 Measurements Using a Palladium Purifier", 1993 Proceedings: Institute of Environmental Sciences 50-54, both of which are incorporated herein by reference).
Getter alloy formulations have been used to remove impurities in hydrogen. In getter purification systems a metal alloy formulation reacts irreversibly with impurities present in hydrogen gas, leaving a purified hydrogen output. Purification is typically carried out in heated packed bed type reactors at temperatures of from 350.degree. C. to greater than 500.degree. C. These systems remove oxygen, water vapor, and nitrogen present in feed gas in amounts up to ppm quantities. However, at these high temperatures methane is created either from conversion of CO or CO.sub.2 to CH.sub.4, or from conversion of H.sub.2 to CH.sub.4, resulting in high methane levels of impurities in the purified hydrogen.
A variation of the getter purification method utilizes special adsorption beds placed upstream from a getter purifier. The system works on the theory that the special adsorption beds will remove carbon dioxide and carbon monoxide, thereby reducing the carbon concentration reaching the getter bed.